The present invention relates to a system and method for depassivating a passivated lithium battery in a battery powered microprocessor controlled device.
More and more, lithium batteries are being utilized to power portable microprocessor controlled devices. As their use has grown, so has a recognition that lithium batteries are subject to a voltage delay often of an undesirable value. Strictly speaking, voltage delay is the time required for a battery to reach an acceptable voltage or power delivery condition after the application of a load. Lithium batteries are subject to voltage delays, often as long as several seconds, because the lithium anode within such batteries is subject to passivation with time. That is, salt crystals build up on the lithium surface of the anode which increase the internal resistance of the battery and reduce its response characteristics. Passivation in lithium batteries seems to be time and temperature sensitive. The longer a lithium battery is stored at elevated temperatures and the longer the period of time between the application of electrical loads to a lithium battery as by a turn on of a device in which the battery is situated, the greater the passivation and the greater the delay time.
Of course, any such voltage delay is undesired. In some cases, an undesired voltage delay just leads to user annoyance, having to wait longer than expected for a lithium battery powered device to respond after turn on. In other cases, particularly where the battery powered device is microprocessor controlled, an excessive voltage delay can be more than annoying, it can result in the device assuming undesired operating conditions in response to misinformation processed by the microprocessor, namely that the battery is dead or in such a state of age or power delivery condition that it requires replacement Such a condition can arise in battery powered microprcessor controlled devices designed to monitor for low battery conditions and if found to be present, issuing a signal to the microprocessor disabling keyboard and other functions of the device under microprocessor control.
In the past the problem of undesired voltage delay associated with lithium batteries has been addressed by battery manufacturers chemically coating the lithium surfaces of the electrodes in their lithium batteries. In practice, this has not represented a total solution to the problem. Still, significant numbers of lithium batteries are subject to undesired passivation and its associated voltage delay particularly when such batteries are stored at elevated temperatures for extended periods of time or are installed in battery powered devices and then used only infrequently.